It is known that in the space industry, it is desirable to miniaturize the 3D imaging optoelectronic modules while using larger optoelectronic sensors having a greater resolution, and while reducing the cost of the device.
FIG. 1 presents a conventional design of an optoelectronic device used in space imaging. It comprises, arranged according to an optical axis 103:
an image-forming optical device 100 with lenses 101 and a camera objective 102 and
a photosensitive sensor 200.
FIG. 2 shows in more detail a photosensitive optoelectronic sensor 200. It comprises an active part 201 such as a silicon chip bonded in a package 203, for example of ceramic, which is the material generally used for space applications. The reference plane of the sensor is in most cases the rear face of the package 203. Electrical connections 204 in the form of PGA (Pin Grid Array) pins make it possible to ensure the connection between the chip and the outside of the package such as a PCB (Printed Circuit Board) circuit. The package is covered with a glass 202 glued onto the package 203.
The camera objective 102 must be perfectly aligned with the chip 201; it must be at right angles to the active surface of the chip and centred on this active surface. The centring accuracies demanded are of the order of 35 μm. This centring step is done manually and is followed by optical measurement phases. This step is lengthy and difficult and requires specific tools and qualified personnel. It is difficult to very accurately centre the camera objective on the chip because it is itself not very well centred in its package. FIGS. 3a and 3b give an idea of the positioning inaccuracies that appear in the step of gluing the chip in its package. The chip 201 can be offset in the plane XY as illustrated in FIG. 3b and/or exhibit an error of perpendicularity in relation to the optical axis 103 for example because of a variable thickness of glue 205 as illustrated in FIG. 3a. Errors of 150 μm and 80 μm, or even more, are commonly observed. Once the sensor 200 is fabricated, the chip 201 is no longer accessible and its positioning can no longer be rectified. The result thereof is that the positioning accuracy of the photosensitive chip does not comply with the desired final accuracy.
One of the problems for space use is also keeping the sensor at a low temperature. The performance levels of an optical sensor become degraded very quickly when the temperature increases. It is mainly the dark current which increases and in actual fact the black becomes grey which is a nuisance in space applications for which black is predominant in most of the images. This problem is amplified by the use of sensors having increasingly greater resolution and therefore dissipating more power.
The solutions currently used to cool the sensors are the addition of a Pelletier heat exchanger and a radiator for dissipating and for transmitting the calories. Over and above the high cost of this exchanger+radiator, the implementation thereof is difficult because the exchange surface of the chip is its bottom face by which it is glued. In addition, given the bulk of this assembly, the printed circuit board to which the sensor is connected is remotely sited, which has drawbacks. In effect, the separation between the sensor and the electronic components of the printed circuit board induces electronic noises.